The present invention relates generally to metering rolls for depositing measured amounts of liquid as a coating on a substrate and, in particular, to metering rolls used for depositing printing ink, protective varnish, or other coating liquid onto moving objects such as metal cans.
It has previously been known to provide metering rolls in the form of a cylindrical metal roll which is mechanically engraved to form a plurality of spaced liquid-container depressions or cells in the outer surface of such roll as a fine pattern of, for example, 250 to 600 cells per inch having a depth of about 12 microns. The metering roll deposits a measured amount of ink or other coating liquid contained in such cells directly on the object to be coated or on an intermediate printing roll which transfers the coating liquid onto such object. In order to improve the wear characteristics of such engraved metal metering rolls, it has been previously proposed to provide a thin chromium plating over the engraved surface of the metal roll which plating is of a greater hardness and, therefore, increases the time between surface refinishing. An example of such an engraved chrome-plated metal metering roll is found in U.S. Pat. No. 3,613,578 of C. R. Heurich issued Oct. 19, 1971. Unfortunately, such finely engraved metal metering rolls still have a short life, due to rapid wear and corrosion, which requires that the surface of the roll be refinished frequently by a complicated and expensive engraving process. This refinishing can only be done by expert engravers and takes a considerable amount of the time to accomplish.
It has also been proposed to provide a plasma flame-sprayed ceramic coating over the engraved surface of a metal metering roll to provide an improved wear surface for such roll as shown in U.S. Pat. No. 4,009,658 of C. R. Heurich, issued Mar. 1, 1977. Unfortunately, such ceramic coating largely fills the engraved depressions or metering cells on the surface of the roll so that the capacity of such cells is only on the order of about one-fifth the volume of the uncoated cell. In addition, the flame-sprayed ceramic coating is deposited in a nonuniform manner with random-sized pores and varying density so that the thickness of such coating varies, which causes the shape of the metering cells to vary in an unpredictable fashion. As a result, the engraved pattern cannot be as fine and is limited to about 90 cells per inch maximum. Another problem with such flame-spray ceramic coated engraved metal metering rolls is that the ceramic coating must be thin and in some locations is permeable to the coating liquid which may be highly corrosive to the underlying metal roll. In these cases, the corrosive ink or other coating liquid attacks the interface between the metal roll and the ceramic coating, thereby separating the coating in places which requires refinishing.
Attempts have been made to solve the corrosion problem associated with flame-sprayed, ceramic-coated, engraved metal metering rolls by providing a resin sealing material in the pores of the ceramic coating. This prevents the corrosive ink or other liquid from being transmitted through the ceramic coating to its interface with the metal roll, as shown in U.S. Pat. No. 4,301,730 of C. R. Heurich and W. A. Runck, issued Nov. 24, 1981. It has also been proposed to provide a film of Teflon plastic over a porous flame-sprayed ceramic coating on a metal roll used in the manufacture of paper, as shown in U.S. Pat. No. 3,942,230 of T. E. Nalband, issued Mar. 9, 1976.
A more recent development is the use of a laser beam to engrave the surface of a ceramic-coated metal metering roll to provide the liquid containing depressions or cells on the surface of such roll after the flame-sprayed ceramic coating is deposited. However, this has the disadvantage that it requires computer-controlled laser equipment which is extremely expensive. In this regard, see the article entitled "Laser-Engraved Anilox Rolls Offer Accuracy, Uniformity," by C. R. Heurich, published by Pamarco, Inc., of Roselle, N.J. This laser engraved ceramic surface is also used by Union Carbide for their "Ucarlox" brand metering rolls.
All of the above-identified prior metering rolls have the common disadvantage that the coating liquid containing depressions or metering cells must be formed by engraving a pattern on the surface of the roll and such engraved surface requires periodic refinishing. This surface refinishing involves mechanically engraving or laser etching a fine pattern of cells at a considerable cost and resulting delay in the use of such rolls. The present invention overcomes these disadvantages by eliminating the need for such engraved surface and instead using as the metering cells the pores of an unengraved metering roll made of porous ceramic material. The porous ceramic material is made in a controlled manner to have pores of substantially uniform, predetermined size distributed substantially uniformly in a predetermined amount throughout such roll. As a result, when the present metering roll needs to be refinished, its outer surface is simply ground to remove the old surface layer, thereby exposing the pores of the porous ceramic material in a new surface layer. Then the resurfaced roll is put back into service without the need for any engraving or etching. This can be done by unskilled labor on site at the user's plant with conventional grinding equipment in a short period of time with resulting cost advantages to the user.
The pores of the porous ceramic material for the present metering roll may be formed by providing filler particles of organic material of a predetermined size which are mixed in a predetermined amount throughout the ceramic base material before molding the roll body. Then, when the roll body of green ceramic material is heated to sinter the ceramic particles together at high temperature, the organic filler particles are burned out at a lower temperature, leaving pores of a predetermined, substantially uniform size and substantially uniform distribution. Thus, the size of the pores can be controlled and varied over a wide range, such as 3 to 100 microns, depending upon the coating liquid which is to be deposited by such metering roll. For example, when depositing a film of coating liquid or printing ink containing a large amount of solvent which evaporates, pores of larger size would be used so that after evaporation of the solvent the deposited coating is of the desired thickness. However, in depositing a thin film of coating liquid which employs mostly solids and only a small amount of such solvent, smaller pores can be employed. Thus, the pore size and distribution may be controlled by changing the size of the organic filler particles and the amount of such filler particles mixed with the ceramic base material used in forming the porous ceramic roll.